Precocious Earth

January 17, 2001 -- Scientists are drawing a portrait
of how Earth looked soon after it formed 4.56 billion years ago,
based on clues within the oldest mineral grains ever found.

Tiny zircons (zirconium silicate crystals) found in ancient
stream deposits indicate that Earth developed continents and
water -- perhaps even oceans and environments in which microbial
life could emerge -- 4.3 billion to 4.4 billion years ago, remarkably
soon after our planet formed.

Right: Where the newly-discovered
zircons fit in Earth history. (Complete
timeline) Image by Dan Brennan.

The findings by two research groups, one in Australia and
the other in the United States, suggest that "liquid water
stabilizes early on Earth-type planets," said geologist
Stephen
Mojzsis, a member of the NASA Astrobiology Institute's University
of Colorado, Boulder, team. "This increases the likelihood
of finding life elsewhere in the universe" because conditions
conducive to life can evidently develop faster and more easily
than once thought.

It also "gives us a new view of the early Earth, where
the Earth cooled quickly" after gas and dust in the newborn
solar system congealed to form planets, said geologist William
Peck, of Colgate University in Hamilton, New York. "There
were continents and water really early -- and maybe oceans and
life -- all to be obliterated later by meteorites, with almost
no record left except these zircons."

Now, the zircons from Western Australia demonstrate that continents
and water existed 4.3 billion to 4.4 billion years ago. "Life
could have had the opportunity to start 400 million years earlier
than previously documented," Mojzsis said.

"Life could have arisen many times, only to be smashed,
and it only gets a hold once the meteorites taper off,"
Peck added.

Mojzsis and Peck belong to separate research teams, one that
found a 4.4-billion-year-old zircon in 1999 and another team
that unearthed a pair of 4.3-billion-year-old zircons last year
from the same area of Western Australia's Jack Hills rock formation.
Both groups published their studies
in the Jan. 11, 2001, issue of the British journal Nature.

The 4.4-billion-year-old zircon is "our earliest record
of the earliest crust" on Earth, Peck said. That zircon
and the slightly younger zircon grains measure roughly 250 microns
wide -- less than one one-hundredth of an inch.

"These
zircons have really been through the wringer," said Peck.

Left: The Jack Hills region of Western
Australia, where the zircons were discovered. Photo by Simon
Wilde.

Their history began sometime after Earth formed, when "liquid
water interacted with rocks," he said. That interaction
can happen in one of three ways: when water exchanges with minerals
in rocks, when crystals grow out of solution in ground water,
or when mineral veins are deposited. Exposure to water increased
the rocks' normally low ratio of the uncommon isotope oxygen-18
to the more-common isotope oxygen-16, he said.

Later, the rocks were melted underground -- or perhaps during
a meteorite bombardment -- and the zircons formed as crystals
within molten granite that was cooling to form solid rock.

The zircon-laden granite eventually was thrust upward to form
mountains, which later eroded. The granite vanished, but the
zircons ultimately came to rest 3 billion years ago in sandy
Australian stream sediments. These sediments later hardened into
rocks that subsequently were altered by heat and pressure.

Both research teams used instruments called ion
microprobes to date and analyze the zircon crystals, which
often contain uranium, rare earth elements and other impurities.
Uranium decays to lead at a known
rate. Uranium-lead ratios in the zircons showed they formed
as early as 4.4 billion to 4.3 billion years ago when they crystallized
in molten granite.

Below: Microscopic view of a zircon
(zirconium silicate) crystal determined to be 4.4 billion years
old. Photo by John W. Valley

Continental
crust is different than crust that underlies the oceans. Granite
is a common rock in continents. And zircons commonly crystallize
in granite.

So the zircons indicate granite was present 4.3 billion to
4.4 billion years ago, while the granite means continents existed
at that time. Such old granitic rock has not been found; it all
has subsequently been eroded away or otherwise recycled. The
ancient zircons are surviving vestiges of crustal granite from
Earth's early years.

"The fact you have a 4.4-billion-year-old zircon from
granite suggests there had to be the rock of the continental
crust," said geologist Sam Bowring of the Massachusetts
Institute of Technology.

Ion microprobe analysis of rare-earth elements within the
zircon crystals also found levels typical of continental rocks,
Peck said.

The presence of water on the young Earth was confirmed when
both groups analyzed the zircons for oxygen isotopes and found
the telltale signature of rocks that have been touched by water:
an elevated ratio of oxygen-18 to oxygen-16.

As a result, "we know there was liquid water at some
point before 4.4 billion years ago," Peck said. Liquid water
had to collect somewhere, raising the possibility of oceans,
he added.

He said it also is likely oceans existed because "to
make continents, you need to have water."

Peck said that before there were oceans, giant plates of Earth's
crust already could have started moving and colliding with each
other, causing large blocks of rock to dive downward in a process
called subduction. Without oceans, that rock could not have melted
to form continental rock like granite, he said.

Below: Outcrop of the type of rock
where the zircons were discovered. The hammer shows scale. Photo
by Simon Wilde.

Once
there were oceans, however, seawater would have reacted with
and hydrated lava erupting from undersea volcanoes at the mid-ocean
ridges. The lava would then have cooled and formed new seafloor,
which later subducted. The water trapped in minerals within the
sinking rock lowered its melting point, triggering volcanic eruptions
that probably produced island chains made of granitic rocks.
It is thought that such "island arcs" ultimately clumped
together to form continents.

"Oceans, atmosphere and continents were in place by 4.3
billion years ago," said Mojzsis.

According to Peck, the first oceans might have formed from
water brought to Earth by comets or have been emitted during
early volcanic eruptions from what became mid-ocean ridges.

The zircons suggest that life could have existed on Earth
4.3 billion years ago, said Mojzsis, because three key factors
necessary for life to take hold were present: energy, organic
material (from
incoming comets and atmospheric reactions) and -- according
to the zircons -- liquid water.

Credits: Discovery of the 4.4-billion-year-old zircon was
reported by Peck, Simon Wilde at the Curtin Institute of Technology
in Australia; John Valley at the University of Wisconsin, Madison;
and Colin Graham of the University of Edinburgh in the United
Kingdom. Wilde found the 4.4-billion-year-old grain in 1999 while
dating zircons from a rock collected in 1984, Peck said. Mojzsis
and colleagues say they found a pair of 4.3-billion-year-old
zircons last year from the same area of Western Australia's Jack
Hills rock formation. Mojzsis worked with geochemist Mark Harrison
of the University of California, Los Angeles, and Robert Pidgeon
of the Curtin Institute of Technology.

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